The specific delivery of molecules to target cells has eluded medical science for decades while the need for such tools has grown. There are a number of mechanisms to deliver molecules into cells (eg. liposomes, viral vectors and polyplex reagents) but these are limited in a number of ways, most notably their inability to selectively deliver molecules in a heterogenous cell population, as would be found in human organs and tissues. In order to deliver molecules safely and effectively, an alternative medical tool needs to be developed that is not only non-immunogenic but capable of selectively identifying target cells in a mixed population. Aptamers have shown potential to fulfil this need.
Aptamers are artificial nucleic acid molecules that can be isolated to bind to an array of macromolecules (Reviewed in Khati (2010)). The selection and design of highly selective aptamers with high affinity to their target has lead to the development of a variety of aptamers capable of binding an array of molecules. The pioneering work by the Gold (Tuerk, 199) and Szostak (Ellington, 1990) groups, identified an in vitro method for the selection of aptamers specific to organic dyes and T4 DNA polymerases, respectively. This method, called Systematic Evolution of Ligands by Exponential enrichment (SELEX), has been adapted to streamline the selection process of aptamers. Aptamers have been developed that specifically target the HIV surface glycoprotein gp120 and inhibit viral entry (Khati, 2003; Zhou, 2009) Making use of their target specificity, these aptamers have been identified as delivery vehicles for targeted delivery of Dicer substrate siRNA to specific cells (Zhou, 2009; McNamara, 2006). Short interfering RNA (siRNA) are small RNA molecules that act with a number of accessory molecules to post transcriptionally silence target genes. These powerful silencing molecules are only active once in the cytoplasm and are not able to internalise efficiently alone. As such, they require active delivery into cells. Aptamers have been identified as efficient delivery agents for not only siRNA but nanoparticles as well (Dhar, 2008). Recently, an anti-gp120 aptamer and anti-tat/rev siRNA chimera was shown to reduce viral replication and helper CD4+ T cell depletion in humanized mice (Neff, 2011). The chimera did not elicit an interferon response, unlike what has been seen with liposome or polyplex regent mediated delivery of siRNA (Zhou, 2009).
Current HIV targeted siRNA-aptamer conjugates are directed against infected lymphocytes by targeting HIV glycoprotein (gp120) residues on the infected cell membrane (Zhou, 2009). The CD4 receptor in lymphocytes/monocytes has also been used to target aptamer-siRNA chimeras for internalization (Wheeler, 2011). However, the CD4 aptamer was not selected for internalization and is inefficient, working in the 1-4 μM range ex vivo.
CD7 is a pan-leucocytic receptor expressed on progenitors of T and B lymphocytes, natural killer cells and dendritic cells (Hao, 2001; Sempowki, 1999) that plays an accessory role in T cell activation (Lazarovits, 1994; Stillwell, 2011) and persists on the surface of mature CD4+ cells (Cotta, 2006; Lobac, 1985). CD7 has been widely studied as a target for delivery of cytotoxic molecules for leukaemia and lymphoma treatment (Peipp, 2002; Bremmer, 2006; Franker, 1997; Vallera, 1996; Waurzyniak, 1997). Previously it was shown using single-chained monoclonal antibodies conjugated to siRNAs that by targeting CD7 for delivery of HIV therapeutics (siRNAs), there is a protective inhibition of viral infection (Kumar, 2008). The study by Kumar (2008) used an antibody for targeting drug delivery, which required an elaborate and complicated conjugation method for attaching the effector molecule.
There is therefore still a need for a method of delivering a target molecule to cells which express a CD7 cell-surface receptor which overcomes at least some of the problems described above.